Analytical procedures commonly require the preparation of dilutions of small quantities of liquids. The diluting operation can be performed manually utilizing calibrated pipettes, burettes, or the like for measuring a precise volume of liquid to be diluted. However, such manual procedures not only are time-consuming, but also give rise to errors in precision and accuracy.
Automated apparatus or equipment for drawing liquid from a source container, measuring a precise quantity of the liquid, and then directing the measured quantity of liquid to a definite location has been provided. Such apparatus or equipment frequently is mechanically cumbersome and complex, and thus subject to breakdown and additionally operates in a manner believed inherently subject to error.
Specifically, some liquid dispensing devices operate by aspiration of liquid drawn through the length of a long feed tube. A miniscus of some shape is formed in the tube at the top of the liquid column, and its shape affects the quantity of liquid that is aspirated at the beginning and end of the aspirating cycle. When only small quantities of liquid are aspirated, the miniscus itself can generate as much as 3 to 4% error in the measured quantity of liquid, which frequently is not acceptable. Moreover, there are response delays in drawing the liquid up the feed tube at the start of the aspirating cycle that create inaccuracies which are amplified for small quantity measurements. Further, the inability to aspirate certain liquids can limit the overall suitability of such dispensing devices.
Other liquid dispensing devices operate by drawing the liquid through the feed tube and into a calibrated cylinder, where the suction force for moving the liquid is generated by moving a piston in the cylinder itself. The reverse movement of the piston subsequently discharges the measured quantity of liquid. However, suction again acts to draw the liquid into the measuring cylinder, which allows for voids or cavities in the measured liquid in the cylinder. This difference between the actual volume of liquid in the cylinder and the volume of the cylinder itself generally is small, but it is amplified when extremely small quantities of liquid are to be measured and dispensed.
An additional problem experienced in both the aspirator and piston types of liquid dispensing devices is the sample carryover from one operating cycle to the next. Where only small quantities of liquid are to be measured and dispensed, the effects of such carryover again can be amplified to reduce the reliability of the measurement and thus the dilution formed from the measured liquid.